1. Field of the Invention
The invention relates to a method of producing starch decomposition products having a narrow molecular weight distribution by splitting starch or starch derivatives.
Starch decomposition products, especially hydroxyethyl starch (HES), play an important role in many areas of medicine, where they are chiefly used as plasma expanders, but they are also used in dialysis (peritoneal dialysis).
In addition, starch decomposition products are administered for dietetic purposes.
2. Description of the Prior Art
Up until now, both hydrolytically decomposed as well as amylase-decomposed starches have been used to produce HES. For example, an enzymatic method is discussed in DE-C 33 13 600.
The above-mentioned chemical or biochemical methods yield products having a broad molecular weight distribution with a considerable proportion of low-molecular compounds, such as glucose, maltose or oligosaccharides; and by-products occur, for example, in the form of sodium chloride or the enzymes used in the process. These undesired components must then be removed in further process steps, e.g. by precipitation with organic solvents (acetone, isopropanol), or by ultrafiltration. On the one hand this is cost-intensive, and on the other it reduces the yield. However, such purification operations are necessary because usually the aim is to produce a product with the narrowest possible molecular weight distribution and a high degree of purity.
DE-A-41 32 701 describes such a method in which starch or starch derivatives are treated with ultrasound in an aqueous mixture (dispersions, suspensions or solutions). The purpose is to adjust the desired mean molecular weight at the desired level, as a function of the processing time and the intensity of exposure to the ultrasonic radiation, while obtaining a very narrow molecular weight distribution, in the practically total absence of undesired low-molecular components.
However, this latter method is technically very complex and in addition requires a high input of energy to split the starch.
DE-A-33-04 775 describes a method for the depolymerization of polysaccharides which can be used only for solutions of polysaccharides having a helix structure, but not for solutions of polysaccharides having a single-chain structure or an aggregated conformation. Furthermore, it has been discovered that ultrasonic depolymerization is not suitable for depolymerization of large batches on an industrial scale.